The baseline localizer control law of an aircraft autolanding control system utilizes lateral deviation and lateral deviation rate values as primary feedbacks to determine, and then control, the position of the aircraft relative to a runway. Lateral deviation values are based on the angle of the localizer beam produced by the localizer transmitter and the estimated distance to the runway threshold, which is a function of the glideslope beam error and radio altitude. Contemporary glideslope beams have a coverage angle of 0.75.degree. and the contemporary radio altimeter range of a glideslope beam is 2,500 feet. Frequently, aircraft "capture" the localizer beam well outside of these glideslope ranges. Out-of-range localizer beam capture often leads to an underestimate of the distance to the runway threshold and, thus, an erroneous estimate of the actual lateral deviation of the aircraft. Errors of up to fifty percent (50%), 20 miles from the runway threshold, can occur. Incorrect lateral deviation estimation can lead to poor localizer beam capture performance, including localizer standoffs and loose tracking of the runway centerline.
One obvious way of improving lateral deviation estimation is to increase the range of the Instrument Landing System (ILS) and Radio Altimeter (LRRA) beam, i.e., increase the range of the glideslope beam. Unfortunately, this solution involves upgrading all ground station ILS transmitters and all airplane radio altimeters. Such a solution is undesirable because it is both time consuming to implement and expensive. The present invention is directed to providing an alternative way of determining the actual lateral deviation from the centerline of a runway of an aircraft located outside the region normally covered by the glideslope beam that does not require a ground station ILS transmitter and/or aircraft radio altimeter modification.